Display device

ABSTRACT

The purpose of the present invention is to improve reliability of the TFT of the oxide semiconductor. The feature of the invention is: A display device comprising: a substrate including a display area where plural pixels are formed, the pixel includes a first TFT of a first oxide semiconductor, a first gate insulating film is formed under the first oxide semiconductor, a first gate electrode is formed under the first gate insulating film, an interlayer insulating film is formed on the first oxide semiconductor; a drain wiring, which connects with the first oxide semiconductor, and a source wiring, which connects with the first oxide semiconductor, are formed on the interlayer insulating film; the drain wiring or the source wiring is a laminated structure of a second oxide semiconductor and a first metal, the second oxide semiconductor is under the first metal.

CLAIM OF PRIORITY

The present application is a continuation of U.S. application Ser. No.15/895,139, filed on Feb. 13, 2018, and claims priority from JapanesePatent Application JP 2017-064920 filed on Mar. 29, 2017, the content ofeach of which are hereby incorporated by reference into thisapplication.

BACKGROUND OF THE INVENTION (1) Field of the Invention

The present invention relates to a display device comprising TFTs (ThinFilm Transistor) that use oxide semiconductors.

(2) Description of the Related Art

A liquid crystal display device or an organic EL display device usesTFTs for switching elements in the pixels or for the built in drivingcircuits. The TFT uses either one of a a-Si (amorphous Silicon), poly-Si(poly Silicon) or oxide semiconductor as an active layer.

The a-Si has low mobility; consequently, there are some problems to usethe a-Si in the TFTs for the peripheral driving circuits. The poly-Sihas high mobility, which is suitable for the TFTs for the peripheraldriving circuits; however, the poly-Si has some problems for theswitching TFTs in the pixels since it has rather bigger leak current.The oxide semiconductor has low leak current and the mobility is higherthan the mobility of the a-Si; however, it has some problems ofreliability in controlling defects in the semiconductor layer.

The patent document 1 (Japanese patent laid open 2012-15436) disclosesthe structure that the entire of the TFT, which comprises the oxidesemiconductor and gate electrode, is covered by the inorganic insulatingfilm of e.g. aluminum oxide, titanium oxide or indium oxide.

The patent document 2 (Japanese patent laid open 2015-92638) disclosesthe structure to suppress the gate leak caused by the tunnel effect whenthe gate insulating film becomes thin in order to improve thecharacteristics of the TFT formed by the oxide semiconductor. The patentdocument 2 discloses to use the material of high dielectric constant ase.g. hafnium oxide, tantalum oxide laminated with silicon oxide, siliconnitride or aluminum oxide, etc. for the gate insulating film.

The patent document 3 (WO 2010/041686) discloses to sandwich the channelof the oxide semiconductor by the inorganic insulating film to stabilizethe characteristics of the TFT formed by the oxide semiconductor. Thepatent document 3 discloses to use e.g. aluminum oxide, titanium oxideor indium oxide for the inorganic insulating film.

SUMMARY OF THE INVENTION

Examples of the oxide semiconductors are: IGZO (Indium Gallium ZincOxide), ITZO (Indium Tin Zinc Oxide), ZnON (Zinc Oxide Nitride), IGO(Indium Gallium Oxide), and so on. Since those semiconductors aretransparent, they are sometimes called TAOS (Transparent Amorphous OxideSemiconductor). By the way, for example, The ratio of the components ofIGZO is generally In:Ga:Zn=1:1:1, however, in this specification, IGZOincludes the one that deviated from the above ratio.

The initial characteristics of the TFT using the oxide semiconductor canbe controlled by the amount of oxide in the oxide semiconductor or inthe insulating film that contacts with the oxide semiconductor; however,controlling the reliability is difficult. Specific problem is thatdefects in the insulating layer increase when an amount of oxygen in theinsulating layer is increased. Therefore, conventionally, the initialcharacteristics and the reliability have been in a relation of tradeoff.

Further, there has been a problem as that: even the amount of oxygen iscontrolled initially, the oxygen gradually moves out from the oxidesemiconductor during the product life, consequently, the characteristicsof the TFT change.

The purpose of the present invention is to realize the TFT formed by theoxide semiconductor that satisfies both of the initial characteristicsand the high reliability during the product life.

The present invention solves the above problem; the concrete measures ofthe present inventions are as follows:

(1) A display device comprising: a substrate including a display areawhere plural pixels are formed, the pixel includes a first TFT of afirst oxide semiconductor, a first gate insulating film is formed underthe first oxide semiconductor, a first gate electrode is formed underthe first gate insulating film, an interlayer insulating film is formedon the first oxide semiconductor; a drain wiring, which connects withthe first oxide semiconductor, and a source wiring, which connects withthe first oxide semiconductor, are formed on the interlayer insulatingfilm, the drain wiring or the source wiring is a laminated structure ofa second oxide semiconductor and a first metal, the second oxidesemiconductor is under the first metal.

(2) A display device comprising: a substrate including a display areawhere plural pixels are formed, the pixel includes a first TFT of afirst oxide semiconductor, a first gate insulating film is formed on thefirst oxide semiconductor, a first gate electrode is formed on the firstgate insulating film, an interlayer insulating film is formed on thefirst gate electrode; a drain wiring, which connects with the firstoxide semiconductor, and a source wiring, which connects with the firstoxide semiconductor, are formed on the interlayer insulating film, thedrain wiring or the source wiring is a laminated structure of a secondoxide semiconductor and a first metal, the second oxide semiconductor isunder the first metal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a liquid crystal display device;

FIG. 2 is a cross sectional view, along the line A-A of FIG. 1;

FIG. 3 is a cross sectional view of the display area of the liquidcrystal display device of the first embodiment;

FIG. 4 is a cross sectional view of the first embodiment;

FIG. 5 is a cross sectional view of the second example of the firstembodiment;

FIG. 6 is a cross sectional view of the third example of the firstembodiment;

FIG. 7 is a cross sectional view of the fourth example of the firstembodiment;

FIG. 8 is a cross sectional view of the fifth example of the firstembodiment;

FIG. 9 is a cross sectional view of the second embodiment;

FIG. 10 is a cross sectional view of the display area of the liquidcrystal display device of the third embodiment;

FIG. 11 is a cross sectional view of the third embodiment;

FIG. 12 is a cross sectional view of the second example of the thirdembodiment;

FIG. 13 is a cross sectional view of the third example of the thirdembodiment;

FIG. 14 is a cross sectional view of the fourth example of the thirdembodiment;

FIG. 15 is a cross sectional view of the fifth example of the thirdembodiment;

FIG. 16 is a cross sectional view of the sixth example of the thirdembodiment;

FIG. 17 is a cross sectional view of the fourth embodiment;

FIG. 18 is a cross sectional view of another example of the fourthembodiment; and

FIG. 19 is a cross sectional view of the display area of the organic ELdisplay device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in detail referring to thefollowing embodiments.

First Embodiment

FIG. 1 is a plan view of a liquid crystal display device, which is usedin e.g. the cellar phone, where the present invention is applied. InFIG. 1, the TFT substrate 10, in which plural pixels 93 are formed, andthe counter substrate 40 are adhered by the seal material 80. The liquidcrystal is sandwiched between the TFT substrate 10 and the countersubstrate 40. The display area 90 is formed inside of the seal material80. In the display area 90, the scan lines 91 extend in lateraldirection and arranged in longitudinal direction; the video signal lines92 extend in longitudinal direction and arranged in lateral direction

The pixel 93 is formed in the area surrounded by the scan lines 91 andthe video signal lines 92. In each of the pixels 93, the pixel electrodeand the TFT, which controls the signals that are to be supplied to thepixel electrode, are formed. The TFT substrate 10 is made bigger thanthe counter substrate 40; the portion of the TFT substrate 10 thatdoesn't overlap with the counter substrate 40 is the terminal area. Thedriver IC 95 is installed in the terminal area; the flexible wiringsubstrate 96 is connected to the terminal area to supply signals andpowers to the liquid crystal display device.

FIG. 2 is cross sectional view along the line A-A of FIG. 1. In FIG. 2,the TFT substrate 10 and the counter substrate 40 are overlapped to eachother. The liquid crystal layer is omitted in FIG. 2 since the thicknessof the liquid crystal layer is much thinner than the thicknesses of theTFT substrate 10 and the counter substrate 40. The portion where the TFTsubstrate 10 and the counter substrate 40 don't overlap is the terminalarea where the driver IC 95 is installed and the flexible wiringsubstrate 96 is connected.

Since the liquid crystal is not self-illuminant, the back light 1000 isset at the rear side of the TFT substrate 10. Images are formed bycontrolling the light from the back light 1000 in each of the pixels.Since the liquid crystal controls only the polarized light, the lowerpolarizing plate 510 is adhered to underneath the TFT substrate 10, andthe upper polarizing plate 520 is adhered to on the counter substrate40.

FIG. 3 is a cross sectional view of the display area 90 of the liquidcrystal display device. In FIG. 3, the TFT substrate 10 is formed byglass or resin. The gate electrode 11 is formed on the TFT substrate 10.The gate electrode 11 is made by e.g. Mo, W or alloys of those metals.The gate insulating film 12 is formed covering the gate electrode 11.The gate insulating film 12 is formed by a silicon oxide (herein aftermay be called SiO) film or a laminated film of a silicon oxide film anda silicon nitride (herein after may be called SiN) film. When thelaminated film is used as the gate insulating film 12, the silicon oxidefilm is located to contact with the first oxide semiconductor 13.

The first oxide semiconductor 13 of e.g. IGZO is formed on the gateinsulating film 12. The drain electrode 14 is formed on one side of theoxide semiconductor 13; the source electrode 15 is formed on anotherside, opposing to the one side, of the oxide semiconductor 13. The drainelectrode 14 and the source electrode 15 can be made of the samematerial as the gate electrode 11 or can be made of the same material asthe video signal line 92. The oxide semiconductor 13 becomes conductivewhere the drain electrode 14 or the source electrode 15 contacts sincethe drain electrode 14 or the source electrode 15 absorbs oxygen fromthe oxide semiconductor 13.

The interlayer insulating film 16 is formed covering the first oxidesemiconductor 13, the drain electrode 14 and the source electrode 15.The interlayer insulating film 16 is formed e.g. either by a siliconoxide film, a laminated film of a silicon oxide film and a siliconnitride film, or a laminated film of a silicon oxide film and analuminum oxide (herein after may be called as AlO) film. If thelaminated film is used, the SiO film is set to contact with the firstoxide semiconductor 13.

Through holes are formed in the interlayer insulating film 16 to connectthe drain electrode 14 and the drain wiring 17 at one through hole andto connect the source electrode 15 and the source wiring 18 at anotherthorough hole. AS will be explained later, the feature of the presentinvention is to make each of the drain wiring 17 and the source wring 18in a two layer structure; namely, the second oxide semiconductor layeris set as the lower layer and the metal is set as the upper layer. Thus,oxygen can be supplied to the first oxide semiconductor 13 thatconstitutes the TFT, easily.

The organic passivation film 19 is formed covering the interlayerinsulating film 16, drain wiring 17 and the source wiring 18. Theorganic passivation film 19 is made as thick as 2 μm to 4 μm since ithas also a role as a flattening film. The through hole 24 is formed inthe organic passivation film 19 to connect the pixel electrode 22 andthe source wiring 18, which connects with the source electrode 15 of theTFT.

The common electrode 20 is formed in a solid plane shape on the organicpassivation film 19. The capacitive insulating film 21 of SiN is formedcovering the common electrode 20; the pixel electrode 22 is formed onthe capacitive insulating film 21. The capacitive insulating film 21 isso called because a holding capacitance is formed between the commonelectrode 20 and the pixel electrode 22 via the capacitive insulatingfilm 21. The alignment film 23 is formed covering the pixel electrode 22for an initial alignment of the liquid crystal molecules. The pixelelectrode 22 is stripe shaped or comb shaped in a plan view. When thevoltage is applied to the pixel electrode 22, the line of force asdepicted by arrows in FIG. 3 is generated, whereby the liquid crystalmolecules 301 are rotated, thus the transmittance of the light from theback light is controlled in a pixel.

In FIG. 3, the counter substrate 40 is set to sandwich the liquidcrystal layer 300 with the TFT substrate 10. On the inner side of thecounter substrate 40, the color filter 41 is formed corresponding to thepixel electrode 22 to form the color images. The black matrix 42 isformed between the color filters 41 to improve the contrast of theimages. The overcoat film 43 is formed covering the color filter 41 andthe black matrix 42. The overcoat film 43 prevents that the pigments inthe color filter 41 goes out and contaminates the liquid crystal layer300. The alignment film 44 is formed covering the overcoat film 43.

FIG. 4 is a cross sectional view of the first embodiment of the presentinvention. In FIG. 4, the gate electrode 11 made of metal is formed onthe TFT substrate 10; the gate insulating film 12 is formed over thegate electrode 11. The gate insulating film 12 is made either by a SiOfilm or a laminated film of a SiO film and a SiN film. The first oxidesemiconductor 13 made of e.g. IGZO, constituting the TFT, is formed onthe gate insulating film 12. The thickness of the first oxidesemiconductor 13 is 10 nm to 70 nm.

The drain electrode 14 is formed at one side of the first oxidesemiconductor 13; the source electrode 15 is formed at another side ofthe first oxide semiconductor 13. Oxygen is extracted from the oxidesemiconductor 13 where the drain electrode 14 or the source electrode 15contacts, consequently, the oxide semiconductor 13 becomes conductivewhere the drain electrode 14 or the source electrode 15 contacts.

The interlayer insulating film 16 is formed covering the first oxidesemiconductor 13, the drain electrode 14 and the source electrode 15.The interlayer insulating film 16 is formed e.g. either by a siliconoxide film, a laminated film of a silicon oxide film and anotherinsulating film. Through holes are foiled in the interlayer insulatingfilm 16 to connect the drain electrode 14 and the drain wiring 17 at onethrough hole and to connect the source electrode 15 and the sourcewiring 18 at another thorough hole. The feature of the present inventionis to make each of the drain wiring 17 and the source wring 18 in a twolayer structure; namely, the second oxide semiconductor layers 171, 181are set as the lower layers and the metals 172, 173 are set as the upperlayers. The metals 172, 182 can be formed by the same material as thevideo signal line as e.g. laminated structure of Ti/Al alloy/Ti. Thethicknesses of the second oxide semiconductors 171, 181 are e.g. 1 nm to30 nm.

A necessary amount of oxygen must be maintained in the first oxidesemiconductor 13 to stabilize the characteristics of the TFT. If theoxygen is supplied from the SiO constituting the interlayer insulatingfilm 16, the SiO must have many defects. If the SiO has a lot ofdefects, however, the defects absorb several gases during themanufacturing processes. The absorbed gases are discharged during thelife time of the product, and absorbed by the first oxide semiconductor13; consequently, the characteristics of the TFT become unstable.

In this invention, as described in FIG. 4, the drain wiring 17 and thesource wiring 18 cover the TFT, which includes the first oxidesemiconductor 13. Thanks to the structure, the oxygen can be easilysupplied to the first oxide semiconductor 13 from the second oxidesemiconductor 171, 181 through the interlayer insulating film 16. Inaddition, the oxygen is easily confined in the interlayer insulatingfilm 16 or in the layers under the interlayer insulating film 16. As aresult, the amount of oxygen is stably maintained in the first oxidesemiconductor 13 without increasing defects in the interlayer insulatingfilm 16. In the meantime, in an example of FIG. 4, both of the drainwiring 17 and source wiring 18 are made two layer structures; it ispossible to make either one of the drain wiring 17 and source wiring 18,which covers more area of the first semiconductor 13, can be made twolayer structure.

FIG. 5 is an example that the interlayer insulating film 16 is made bytwo layers of the silicon oxide film 161 and the aluminum oxide film162. In this structure, the aluminum oxide 162, too, can be a source ofoxygen; in addition, the aluminum oxide 162 can have a role to confinethe oxygen at the side of the first oxide semiconductor 13. Further, thealuminum oxide has a good blocking structure, thus, it prevents moisturefrom intruding in the first oxide semiconductor 13.

FIG. 6 is an example that the interlayer insulating film 16 is made bytwo layers of the silicon oxide film 161 and the silicon nitride film163. In this structure, the silicon oxide film 161 contacts with thefirst oxide semiconductor 13; the silicon nitride film 163 contacts withthe second oxide semiconductors 171, 181. The silicon nitride film 163is a superior blocker against the moisture. Consequently, the two layerstructure of the silicon oxide film 161 and the silicon nitride film 163makes a superior blocker against impurities. Further this structure hasa merit in manufacturing process that the silicon oxide film 161 and thesilicon nitride film 163 are continuously formed by CVD (Chemical VaporDeposition).

The silicon nitride film 163, however, discharges hydrogen, itdeteriorate the oxide semiconductor 13, which constitutes TFT. In thestructure of FIG. 6, the hydrogen from the silicon nitride film 163 isabsorbed by the second oxide semiconductors 171, 181, which are underlayers of the drain wring 17 and the source wiring 18; thus, theinfluence of the hydrogen to the first oxide semiconductor 13, whichconstitutes the TFT, is suppressed; consequently, the characteristics ofthe TFT can be stabilized.

FIG. 7 is a cross sectional view that shows another example of the firstembodiment. FIG. 7 differs from FIG. 4 in that the gate insulating film12 is formed by two layers of the lower layer 121 and the upper layer122. The lower layer 121 is made of AlO. The upper layer 122 is formedby SiO or a laminated film of SiO and SiN. In this case, too, the SiOfilm contacts with the first oxide semiconductor 13.

According to the structure of FIG. 7, oxygen can be supplied to thefirst oxide semiconductor 13 from AlO, which constitutes the lower layer121 of the gate insulating film 12. At the same time, the AlO layerprevents that the gate electrode 11, which is made of metal, absorbsoxygen from the first oxide semiconductor 13 through the gate insulatingfilm 12. Therefore, the first oxide semiconductor 13 can maintain enoughoxygen without making many defects in the SiO, which constitutes thegate insulating film 12.

FIG. 8 is cross sectional view that shows yet another example of thefirst embodiment. FIG. 8 differs from FIG. 4 in that the gate electrode11 is formed by two layers of the lower layer 111 and the upper layer112. In FIG. 8, the upper layer 112 is formed by a third oxidesemiconductor. The thickness of the third oxide semiconductor 112 is 1nm to 30 nm. The third oxide semiconductor 112 prevents that the gateelectrode 11 absorbs the oxygen from the first oxide semiconductor 13;further, the third oxide semiconductor 112 supplies oxygen to the firstoxide semiconductor 13. Therefore, the first oxide semiconductor 13 canmaintain enough oxygen without making many defects in the SiO film thatconstitutes the gate insulating film 12.

The structures of FIGS. 7 and 8 can coexist. In addition, structures ofFIGS. 4-8 are combinable arbitrarily. Thus, the effect of the inventioncan be further intensified.

Second Embodiment

Since Poly-Si has a high mobility, a high speed TFT can be realized. Onthe other hand, the oxide semiconductor has low leak current; thus, theTFT that uses the oxide semiconductor is suitable to the switchingelement. Therefore, using both of the TFT of the Poly-Si and the TFT ofthe oxide semiconductor can realize a high performance display device.One example is that the TFT of the Poly-Si is used for the drivingcircuit while the TFT of the oxide semiconductor is used for theswitching element in a pixel.

FIG. 9 is a cross sectional view of the second embodiment of the presentinvention where the TFT of Poly-Si and the TFT of the oxidesemiconductor 13 coexist. Such a structure is called a hybrid structure.The TFT of the oxide semiconductor in FIG. 9 is the same as thestructure of FIG. 4. The gate electrode 11 is, however, formed on thesilicon oxide film 72, formed by CVD using TEOS (Tetraethylorthosilicate) as the material, which works as a gate insulating filmfor the Poly-Si TFT.

In FIG. 9, the undercoat 70 is formed on the TFT substrate 10. Thestructure of the undercoat 70 is e.g. a laminated film of SiO/SiN. Theundercoat prevents that the poly-Si 71 is contaminated by impuritiesfrom the TFT substrate, which is formed by glass or resin. The SiO filmis superior in adhering with the substrate 10 while SiN is superior inblocking the moisture.

In FIG. 9, the Poly-Si 71 is formed on the undercoat 70. Poly-Si isformed as that: initially, a-Si is formed; then, excimer laser isapplied on the a-Si to transform it to Poly-Si; after that, the poly-Si71 is patterned. The gate insulating film 72 is foiled covering thePoly-Si 71. The gate insulating film 72 is famed by CVD using TEOS asthe material.

The gate electrode 73 for the TFT of the Poly-Si is formed on the gateinsulating film 72. The gate electrode 11 for TFT of the oxidesemiconductor 13 is formed at the same time. The gate insulating film 12is formed covering the gate electrodes 11 and 73. The processes afterthat are the same as the processes explained for the TFT of the oxidesemiconductor in FIG. 4. The drain wiring 17 and the source wiring 18for the TFT of the oxide semiconductor 13 and the TFT of the Poly-Si 71are formed simultaneously, and the through holes for the drain wiring 17and the source wiring 18 are formed simultaneously. Namely, the drainwiring 17 and the source wiring 18 at the TFT of the Poly-Si 71 are twolayer structures of the second oxide semiconductor 171, 181 and themetal 172, 182. The second oxide semiconductor 171, 181 can beeliminated at the TFT of the Poly-Si.

The structure of FIG. 9 enables a stable production for the displaydevices that use the hybrid structure of high reliability that uses theTFT of the oxide semiconductor and the TFT of poly-Si.

Third Embodiment

The first embodiment and the second embodiment are the cases where thepresent invention is applied to the bottom gate type TFT of the oxidesemiconductor; however, the present invention is applicable to the topgate type TFT, too. FIG. 10 is a cross sectional view of the displayarea of the liquid crystal display device that uses a top gate type TFT.In FIG. 10, the TFT substrate 10 is famed by glass or resin. Theundercoat 70 is formed on the TFT substrate 10 so that the semiconductorlayer is not contaminated by impurities from the glass or resin.

The undercoat 70 is the same as explained in FIG. 9; however, in thisembodiment, the AlO layer may be applied in addition to a laminated filmof the SiO film and the SiN film. When the AlO layer is laminated, theamount of oxygen in the first oxide semiconductor 13 can be maintainedmore stably.

In FIG. 10, the oxide semiconductor 13 of e.g. IGZO is formed on theundercoat 70. The gate insulating film 25 is formed covering the oxidesemiconductor 13. In this invention, as will be explained later, thegate insulating film 25 is formed by SiO; however, the AlO film may belaminated on the SiO film. The gate electrode 26 is formed on the gateinsulating film 25. The first example of the current embodiment, asexplained later, may have a laminated structure of the third oxidesemiconductor and the metal for the gate electrode 26.

In FIG. 10, after the gate electrode 26 is formed, an ion implantationis applied to the oxide semiconductor using the gate electrode 26 as amask to form defects in the oxide semiconductor 13 for conductivity;thus, the drain region 131 and the source region 132 are formed in theoxide semiconductor 13. The interlayer insulating film 16 is formedcovering the gate electrode 26 and the gate insulating film 25. Theinterlayer insulating film 16 is formed by SiO, SiN or AlO as explainedin the first embodiment.

Through holes are formed in the interlayer insulating film 16 and thegate insulating film 25; subsequently, the drain wiring 17 and thesource wiring 18 are formed. The drain wiring 17 connects with the videosignal line 92 and the source wiring 18 connects with the pixelelectrode 22 via through hole 24. As will be explained later, the drainwiring 17 and the source wiring 18 are laminated structures of the metaland the second oxide semiconductor.

The organic passivation film 19 is formed covering the drain wiring 17,the source wiring 18 and the interlayer insulating film 16. The layersabove the organic insulating film 19 is the same as explained in FIG. 3,thus, the explanation is omitted.

FIG. 11 is a cross sectional view that shows the feature of the thirdembodiment. In FIG. 11, the drain wiring 17 and source wiring 18 arelaminated films of the second oxide semiconductors 171, 181 and themetals 172, 182. The thickness of the second oxide semiconductor 171,181 is 1 nm to 30 nm. Thanks to the second oxide semiconductor 171, 181,oxygen is confined in the layer under the interlayer insulating film 16,and further, oxygen is supplied to the first oxide semiconductor 13 fromthe second oxide semiconductor 171, 181 through the interlayerinsulating film 16. Thus, variation in characteristics in the TFT can besuppressed.

FIG. 12 is a cross sectional view that shows another example of thethird embodiment. FIG. 12 differs from FIG. 11 in that the interlayerinsulating film 16 is formed by two layer structure; the lower layer 161is made of SiO and the upper layer 162 is made of the AlO. The thicknessof the AlO film 162 is 1 nm to 50 nm. The AlO has superiorcharacteristics in blocking oxygen, thus, oxygen can be confined in theside of the TFT more efficiently. In addition, the AlO can supply oxygento the first oxide semiconductor 13, which constitutes the TFT.

FIG. 13 is cross sectional view that shows yet another example of thesecond embodiment. FIG. 13 differs from FIG. 11 in that the undercoat 70is formed by a two layer structure; wherein the lower film 701 is a SiOfilm or a laminated film of the SiO film and the SiN film, the upperfilm 702 is an AlO film. The AlO film contacts with the first oxidesemiconductor 13 that constitutes the TFT. The thickness of the AlO filmis 1 nm to 30 nm. FIG. 13 has an additional merit besides the merit ofFIG. 11 that the AlO layer can efficiently supply oxygen to the firstoxide semiconductor 13 since the AlO film directly contact with thefirst semiconductor 13.

FIG. 14 is cross sectional view that shows yet another example of thesecond embodiment. FIG. 14 differs from FIG. 11 in that the gateinsulating film 25 is a laminated layer of the lower layer 251 made ofthe SiO and the upper layer 252 made of the AlO. The lower layer 251made of SiO contacts with the first oxide semiconductor 13 while theupper layer 252 made of the AlO contacts with the gate electrode 26.

The upper layer 252 made of the AlO layer is a source of oxygen for thefirst oxide semiconductor 13. Thus, enough oxygen can be supplied to theoxide semiconductor 13 without increasing defect density in the lowerlayer 251 made of the SiO. Further, the AlO is a superior blockeragainst oxygen; thus, oxygen can be confined efficiently inside of theoxide semiconductor 13. The structure of FIG. 14 has an above explainedmerit in addition to the merit in FIG. 11, thus, a reliability of theTFT including the oxide semiconductor 13 can be improved.

FIG. 15 is cross sectional view that shows yet another example of thesecond embodiment. FIG. 15 differs from FIG. 11 in that the gateelectrode 26 is a laminated layer of the lower layer 261 made of thethird oxide semiconductor and the upper layer 262 made of the metal. Thethird oxide semiconductor layer 261 is formed by e.g. IGZO. The thirdoxide semiconductor 261 can be a different material from the first oxidesemiconductor 13 that constitutes the TFT; however, if the same materialas the first oxide semiconductor 13 is used, the process can be simpler.

In FIG. 15, the third oxide semiconductor 261 contacts with the gateinsulating film 25. The third oxide semiconductor 261 can supply oxygento the first oxide semiconductor 13, which constitutes the TFT; thus,the variation in characteristics in the TFT can be suppressed. Asdescribed above, the structure of FIG. 15 has an above explained meritin addition to the merit in FIG. 11, thus, a reliability of the TFTincluding the oxide semiconductor 13 can be improved.

FIG. 16 is cross sectional view that shows yet another example of thesecond embodiment. FIG. 16 differs from FIG. 11 in that the protectivelayer 50 made of metal is formed on the first oxide semiconductor 13 atportions the drain wring 17 and the source wiring 18 connect with thefirst oxide semiconductor 13. The drain wiring 17 and the source wiring18 are formed in the through holes formed in the interlayer insulatingfilm 16 and the gate insulating film 25. The through holes are formed bye.g. dry etching. Since the oxide semiconductor film 13 is as thin as 10nm to 70 nm, there is a danger that the oxide semiconductor 13 iseliminated simultaneously when the interlayer insulating film 16 and thegate insulating film 25 are etched.

In FIG. 16, the protective layers 50 are formed on the oxidesemiconductor 13 where the oxide semiconductor connects with the drainwiring 17 and with the source wring 18; thus, disappearance of the oxidesemiconductor 13 can be avoided. The metal that constitutes theprotective layer 50 can be the same metal for the video signal line 92,e.g. the structure that Al alloy is sandwiched by e.g. Ti. The structureof FIG. 16 enables to fabricate a TFT of the oxide semiconductor 13having a high reliability.

The structures of FIGS. 11 to 16 are combinable. The combination makesthe characteristics in the TFT of the oxide semiconductor 13 morereliable.

Fourth Embodiment

FIG. 17 is a cross sectional view of the fourth embodiment. FIG. 17 is ahybrid type structure that includes the dual gate type TFT of the oxidesemiconductor and the TFT of the Poly-Si. At the outset, the dual gatetype TFT of the oxide semiconductor at the left hand side is explained.

The ON current of the TFT of the oxide semiconductor is 10 times largerthan the ON current of the TFT of the a-Si; however, it is not big asthe ON current of the TFT of the Poly-Si. The dual gate type canincrease the ON current of the TFT of the oxide semiconductor 13.

The figure of the left hand side of FIG. 17 is a cross sectional view ofthe TFT of dual gate type. In the left hand side TFT of FIG. 17, theundercoat 70 is formed on the TFT substrate 10; the third gateinsulating film 72, which is a gate insulating film 72 of the TFT of thePoly-Si is formed on the undercoat 70; the gate electrode 11 is formedon the third gate insulating film 72; the gate insulating film 12 isformed on the gate electrode 11. The first oxide semiconductor 13, whichconstitutes the TFT, is formed on the gate insulating film 12.

The second gate insulating film 60 is formed on the first oxidesemiconductor 13; the second gate electrode 61 is formed on the secondgate insulating film 60. The interlayer insulating film 16 is formedcovering the second gate electrode 61 and the second gate insulatingfilm 60. Through holes are formed in the interlayer insulating film 16and the gate insulating film 60 to form the drain wiring 17 and thesource wiring 18 in those through holes.

The drain wiring 17 and the source wiring 18 are laminated films of thesecond oxide semiconductor 171, 181 and the metal 172, 182. Thestructures and the functions of the drain wiring 17 and the sourcewiring 18 are the same as explained at FIG. 4 of the first embodimentand at FIG. 11 of the third embodiment. According to the dual gate typeTFT of the oxide semiconductor 13 of the present invention, the TFT ofhigh ON current and high reliability can be realized.

The right hand side of FIG. 17 is a cross sectional view of the TFT ofthe Poly-Si 71. The structure of the TFT of the Poly-Si 71 in FIG. 17 isthe same as the TFT of the Poly-Si 71 in FIG. 9. Therefore, theexplanation of the Poly-Si TFT is omitted. According to the structure ofFIG. 17, the display device having hybrid type TFTs of high reliabilityformed by the TFT of the oxide semiconductor 13 and the TFT of thePoly-Si 71 can be realized.

FIG. 18 is a cross sectional view of another example according to thecurrent embodiment. FIG. 18 differs from FIG. 17 in that the protectivelayer 50 made of metal is formed on the first oxide semiconductor 13 atportions the drain wring 17 and the source wiring 18 connect with thefirst oxide semiconductor 13.

As depicted in FIGS. 17 and 18, the through hole is formed through fivelayers in the TFT of the Poly-Si while the through hole is formedthrough three layers in the TFT of the oxide semiconductor. Thus, theoxide semiconductor 13 is exposed to the etching media longer time thanthe Poly-Si is; consequently, a danger of dissolution of the oxidesemiconductor 13 exists.

In addition, in the TFT of the poly-Si, the through holes must becleaned with hydrofluoric acid HF. At this time, the oxide semiconductor13 is exposed to the hydrofluoric acid HF, too. The oxide semiconductor13 is dissolved by the hydrofluoric acid HF easily.

FIG. 18 is a cross sectional view of the hybrid type TFT that overcomesthe above explained problem. The structure of FIG. 18 can avoid theproblem that the oxide semiconductor is dissolved by the etchingsolution since the protective layer 50, which is formed by metal, isformed on the oxide semiconductor 13. According to the structure of FIG.18, the display device having hybrid type TFTs of high reliabilityformed by the TFT of the oxide semiconductor 13 and the TFT of thePoly-Si 71 can be realized.

Fifth Embodiment

The first to fourth embodiments are explained in regard to the liquidcrystal display device as depicted in FIGS. 1-3. The present inventionis, however, applicable to the organic EL display device as well as tothe liquid crystal display device. FIG. 19 is a cross sectional view ofthe display area of the organic EL display device. In FIG. 19, thefollowing structure is the same as FIG. 10 of the liquid crystal displaydevice; namely, the TFT is formed on the TFT substrate 10; the organicpassivation film 19 is formed on the TFT; the through hole is formed inthe organic passivation film 19.

Therefore, the structure of the TFT of the oxide semiconductor,explained in the first to fourth embodiments, is applicable to theorganic EL display device.

In FIG. 19, the refection electrode 30 is formed on the organicpassivation film 19; the oxide conductive film as ITO (Indium Tin Oxide)for the anode 31 is formed on the reflection electrode 30. The bank 32is formed by e.g. acrylic resin covering the anode 31 and the organicpassivation film 19. In the hole of the bank 32, the organic EL layer 33is formed as a light emitting layer on the anode 31. The organic ELlayer 33 is constituted by plural layers; the thickness is about severalhundred nm even all the layers are combined, namely, each of the layersis very thin. The bank is formed so that the organic EL layer 33 doesn'thave a step disconnection at the edge of the anode 31 or the reflectionelectrode 30.

In FIG. 19, the upper electrode for a cathode 34, which is made of theoxide conductive film as e.g. ITO or IZO (Indium Zinc Oxide), or a thinmetal, is formed over the organic EL layer 33. Since the organic ELlayer 33 is decomposed by moisture, the protective film 35, made of e.g.SiN, is formed to prevent the intrusion of moisture.

Since the organic EL display device uses the reflection electrode 30,the external light is reflected, which deteriorates the visibility ofthe screen. To prevent this phenomenon, the circular polarizing plate 37is adhered to the screen e.g. via the adhesive 36.

As described above, the structure of the organic EL display device hasthe same structure as the liquid crystal display device up to formationof the drain wring 17 and the source wiring 18 with the oxidesemiconductor 13; thus, the present invention, explained by the first tofourth embodiments, is applicable to the organic EL display device, too.

What is claimed is:
 1. A semiconductor device comprising: a substrate, afirst TFT having a first oxide semiconductor, a first gate insulatingfilm formed under the first oxide semiconductor, and a first gateelectrode formed under the first gate insulating film, an interlayerinsulating film is famed on the first oxide semiconductor, a firstthrough hole and a second through hole are formed across the interlayerinsulating film in a direction perpendicular to the substrate, a drainconductor, which connects with the first oxide semiconductor through thefirst through hole, and a source conductor, which connects with thefirst oxide semiconductor through the second through hole, are formed onthe interlayer insulating film, at least one of the drain conductor andthe source conductor is a laminated structure of a second oxidesemiconductor and a first metal, the second oxide semiconductor isbetween the first metal and the substrate, and the laminated structurecontacts a side wall of the first through hole.
 2. The semiconductordevice according to claim 1, wherein the interlayer insulating film is alaminated structure of a first silicon oxide film and a first aluminumoxide film, the first silicon oxide film is under the first aluminumoxide film.
 3. The semiconductor device according to claim 1, whereinthe interlayer insulating film is a laminated structure of a firstsilicon oxide film and a first silicon nitride film, the first siliconoxide film is under the first silicon nitride film.
 4. The semiconductordevice according to claim 1, wherein the first gate insulating film is alaminated structure of a second aluminum oxide film and a second siliconoxide film, the second silicon oxide film is over the second aluminumoxide film.
 5. The semiconductor device according to claim 1, whereinthe first gate electrode is a laminated structure of a second metallayer and a third oxide semiconductor, the second metal layer is underthe third oxide semiconductor.
 6. The semiconductor device according toclaim 1, wherein the substrate includes a second TFT of a poly-siliconsemiconductor, a distance between the poly-silicon semiconductor and thesubstrate is less than a distance between the first oxide semiconductorand the substrate.
 7. A semiconductor device comprising: a substrate, afirst TFT having a first oxide semiconductor, a first gate insulatingfilm famed on the first oxide semiconductor, and a first gate electrodeformed on the first gate insulating film, an interlayer insulating filmis formed on the first gate electrode, a first through hole and a secondthrough hole are formed across the interlayer insulating film in adirection perpendicular to the substrate, a drain conductor, whichconnects with the first oxide semiconductor through a first throughhole, and a source conductor, which connects with the first oxidesemiconductor through a second through hole, are formed on theinterlayer insulating film, at least one of the drain conductor or thesource conductor is a laminated structure of a second oxidesemiconductor and a first metal, the second oxide semiconductor isbetween the first metal and the substrate, and the laminated structurecontacts a side wall of the first through hole.
 8. The semiconductordevice according to claim 7, a protective layer is formed between thefirst oxide semiconductor layer and the drain wiring, and between thefirst oxide semiconductor layer and the source wiring.
 9. Thesemiconductor device according to claim 7, wherein the interlayerinsulating film is a laminated structure of a first silicon oxide filmand a first aluminum oxide film, the first silicon oxide film is underthe first aluminum oxide film.
 10. The semiconductor device according toclaim 7, wherein the interlayer insulating film is a laminated structureof a first silicon oxide film and a first silicon nitride film, thefirst silicon oxide film is under the first silicon nitride film. 11.The semiconductor device according to claim 7, wherein the first gateinsulating film is a laminated structure of a second aluminum oxide filmand a second silicon oxide film, the second silicon oxide film is underthe second aluminum oxide film.
 12. The semiconductor device accordingto claim 7, wherein the first gate electrode is a laminated structure ofa second metal layer and a third oxide semiconductor, the second metallayer is on the third oxide semiconductor.
 13. The semiconductor deviceaccording to claim 7, wherein the substrate includes a second TFT of apoly-silicon semiconductor, a distance between the poly-siliconsemiconductor and the substrate is less than a distance between thefirst oxide semiconductor and the substrate.
 14. The semiconductordevice according to claim 8, wherein the substrate includes a second TFTof a poly-silicon semiconductor, a distance between the poly-siliconsemiconductor and the substrate is less than a distance between thefirst oxide semiconductor and the substrate.
 15. The semiconductordevice according to claim 7, a second gate insulating film including athird silicon oxide film, a second gate electrode is formed under thesecond gate insulating film, the first oxide semiconductor film isformed on the third silicon oxide film.
 16. The semiconductor deviceaccording to claim 15, the second gate insulating film is a laminatedfilm of the third silicon oxide film and a third aluminum oxide film,the second gate electrode contacts with the third aluminum oxide film.17. The semiconductor device according to claim 15, the second gateelectrode is formed by a laminated film of a second metal layer and athird oxide semiconductor film, the third oxide semiconductor film is onthe second metal layer.
 18. The semiconductor device according to claim8, a second gate insulating film including a third silicon oxide isformed under the first oxide semiconductor, a second gate electrode isformed under the second gate insulating film, the first oxidesemiconductor is formed on the third silicon oxide film.
 19. Thesemiconductor device according to claim 15, wherein the substrateincludes a second TFT of a poly-silicon semiconductor, a distancebetween the poly-silicon semiconductor and the substrate is less than adistance between the second gate electrode and the substrate.
 20. Thesemiconductor device according to claim 18, wherein the substrateincludes a second TFT of a poly-silicon semiconductor, a distancebetween the poly-silicon semiconductor and the substrate is less than adistance between the second gate electrode and the substrate.